1. Field of the Invention
The present invention relates to the field of plasma processing and, more particularly, to the use of plasma equipment for modification of materials.
2. Prior Art
Plasma processing equipment is used extensively in the industry for the modification of materials. These modifications include etching and deposition of films for fabrication of microelectronic circuits and semiconductor devices. The modifications also may include implantation of chemical species that change the friction and wear properties of surfaces.
A plasma is a gas (or a gas mixture) which is energized so that it is partially decomposed into species that are electrically charged. A variety of techniques are known for energizing the gas. One commonly used technique is the energizing of the gas by imposing an electric field on the gas from an external source. A common practice is to use high frequency alternating-current (AC) fields to energize or excite the gas. For example, radio-frequency (RF) fields are generated at frequency ranges near 10 MHz. At still higher frequencies, in the order of 1000 MHz, microwave fields are generated. In some instances, these electric fields are utilized in combination with magnetic fields which are used for the purpose of confining the plasma. Electron cyclotron resonance (ECR) plasma processing is one technique for controlling the plasma with the use of electric and magnetic fields.
The plasma is typically retained in a chamber of a processing equipment and isolated from the surrounding ambient and this plasma usually contains species that undergo chemical reactions. The plasma chamber and its gas-handling equipment are typically referred to as a reactor. The source of the electrical power that energizes the plasma is commonly referred to as a generator. Usually, there are a number of components, including cables, wave guides, inductors, capacitors, matching network, tuner and/or an impedance transforming network coupling the generator to the reactor. These components are included in a system sometimes referred to as a coupler or a coupling system. The generator and the coupling system together comprise the AC source that energizes the plasma.
Various schemes have been devised in the prior art for coupling the generator, coupler, and the reactor to operate as a plasma processing system for example, in U.S. Pat. No. 4,824,546 (Ohmi) an RF power source is coupled to a vacuum vessel through a matching circuit in order to provide a sputtering apparatus for forming an insulating thin film. Band reject filters are provided to permit only high frequencies to be supplied.
Another example is disclosed in U.S. Pat. No. 4,579,618 (Celestino et al.) in which two power sources are each coupled through a matching network to a plasma reactor. A filter/combiner is coupled between the low frequency power supply and the high frequency power supply. The filter/combiner serves three purposes which are unique to a single electrode, dual frequency plasma reactor. The power of each power supply must be largely prevented from reaching the other power supply, the mixing products caused by the coupling of two different frequencies to a non-linear load must be attenuated and the radiation emitted by the reactor and the various interconnections must be minimized.
A key feature of most plasmas, is that the plasmas have "non-linear" impedance characteristics. Non-linearity is a mathematical definition signifying that the magnitude of the voltage (electric field) in the plasma is not directly proportional to the magnitude of the current (magnetic field). Typically, the generators employed in various plasma systems are designed to generate an output of predominantly single-frequency. However, because of the non-linearity of the plasma, signals at multiples of the fundamental generator frequency are generated by the plasma. These multiple frequencies of the fundamental frequency are called harmonic frequencies (or harmonics). The amplitude of the harmonics affect certain properties of the plasma, such as direct current (DC) bias, which impact the particular plasma process. The amplitude of the harmonics is determined by the interaction of the plasma with the generator and the coupling system and is difficult to control simply by adjusting the amplitude of the fundamental frequency component.
Plasma non-linearity is a phenomenon which plays an important role in causing the plasma conditions to be dependent upon the electrical characteristics of the generator, as well as the coupling system, at both the operating (fundamental) frequency of the generator and at the various harmonic frequencies. That is, if satisfactory operation of a plasma reactor is achieved for a given generator and coupling system, the parameters of the generator and the coupling system cannot be readily changed without affecting the plasma itself. Thus, generally it is impractical, if not possible, to make changes to the electrical parameters of the generator and/or the coupling system and still be able to reproduce the desired plasma conditions, simply by readjusting the amplitude of the generator output. Typically, what is required is a considerable retuning of the system in order to satisfactorily couple the reactor to the generator and/or the coupling system to obtain the desired plasma conditions.
Therefore, it is difficult to replace a generator from a first manufacturer with a generator from a second manufacturer and obtain the same plasma conditions, unless the electrical parameters of the two generators are identical. Similarly, if a change is made to an impedance matching network in the coupling system, due to a component change for example, the same plasma conditions cannot be reproduced unless the networks are substantially identical. Merely changing the amplitude of the generator output will not compensate for the impedance differences in the generator and/or the coupling system.
An added problem also exists when certain parameters associated with two systems are not substantially identical. If two installations of plasma systems are made using identical generators and reactors but different lengths of coaxial cable (wave guides and/or other transmission mediums) are utilized in the systems then generally identical plasma conditions cannot be reproduced. In actual practice, this lack of reproducibility of desired operating plasma conditions under realistic conditions presents significant difficulties to the user. For example, if an RF generator requires service and/or corrective repair, it can only be replaced with another identical RF generator without undue tuning and adjustment.
Accordingly, it is appreciated that a plasma system which is flexible in design to accommodate a multitude of generator sources, as well as coupling systems, such that the reactor could repeatedly reproduce desirable plasma operating conditions, will provide for an improvement over the prior art.